1. Field of the Invention
The present invention relates to the field of laser systems, and more specifically, it relates to laser systems in which the gain medium is diode-pumped.
2. Background
Lasers have been produced from a variety of materials and in all phases: liquid, gas, plasma and solid-state. Liquid lasers most commonly contain of a gain medium of organic dye salts such as derivatives of Rhodamine or fluorescein which are dissolved in solvents such as methanol or water. The gain medium is excited by light produced typically by flashlamps or another laser. These dye lasers exhibit gain across a broad region of the visible to near infrared spectrum. The high gain cross section and broad tunability make these lasers attractive for a number of uses. On the other hand, these dye lasers exhibit a very short upper-state lifetime and low saturation fluence which makes them unsuitable for high energy applications. Furthermore, the absorption bands of these dyes do not overlap with the emission bands of high power semiconductor diode lasers (typically 780 to 980 nm) and are therefore not amenable to pumping with currently available laser diodes.
There have been various attempts at achieving inorganic liquid lasers composed of rare earth salts dissolved in various solvents. The challenge in these systems has been the tendency of the lasing ion (e.g., Nd3+) to undergo radiationless decay from the upper laser level in most liquid solutions. This de-excitation is due to the high energy vibrations of bonds involving light atoms such as hydrogen. By eliminating the presence of any light atoms, this quenching can be reduced to the point where lasing can be achieved. For example, this quenching has been used to achieve lasing in Neodymium Selenium Oxychloride.
A significant challenge with such liquid lasers is the corrosive nature of the solvents which requires special handling both the preparation and use of the laser medium. When initially developed, these systems were initially pumped by flashlamps. By using recently developed laser diode arrays as pump sources, the pump light can match the absorption spectrum thereby enabling a resurgence of interest in this type of liquid laser.
Solid-state lasers contain a gain medium which is comprised of a lasing ion contained in a crystal or amorphous matrix. The most common lasing species are based on rare-earth elements such as neodymium, erbium, ytterbium, etc. The laser properties (e.g., absorption and emission cross-section, upper-state lifetime, etc.) of the gain medium are determined by the interaction of the local crystal field with the field of the ion itself. This interaction determines the specific energy levels of the ion and their width. For example, the neodymium ion exhibits a series of narrow absorption lines centered around 808 nm and a series of strong emission lines around 1064 nm when bound in the common crystal matrix, yttrium aluminum garnet (YAG). This laser medium is formed when a small amount of neodymium replaces the yttrium ion in the garnet crystal matrix resulting in neodymium-doped YAG (Nd:YAG). By placing the neodymium in other crystal hosts, very different laser properties can be obtained. For example, neodymium dissolved in phosphate glass will produce a series of broad absorption bands extending from 500 to over 900 nm and tens of nanometers in width. Gain is exhibited across a broad band centered at 1054 nm. Unlike dye lasers, these solid-state lasers exhibit a long upper-state lifetime and a high saturation fluence which enables high energy output. In addition, their absorption bands in the 800 to over 950 nm range make them ideal for excitation by the emission from semiconductor laser diodes based on gallium arsenide (e.g., AlGaAs, InGaAs).
Solid-state lasers are formed by growing the crystal (or melting the glass) and then cutting and polishing the crystal into the desired shape. Large scale, single-pulse systems with the solid-state media in the shape of disks have been developed for laser-driven inertial confinement fusion research with disks over 40 cm in diameter. These systems produce very high pulse energy (approximately 15 kJ) but disadvantageously, have low average power (less than 1 W). High average power systems containing either Nd:YAG or Yb:YAG as the gain medium and pumped by laser diodes have been developed at the kilowatt level. A typical diode-pumped solid-state laser is shown in FIG. 2. The solid state laser 200 includes a laser rod 202 surrounded by a flow tube 204 (sheath) which directs a coolant fluid flow 206 across the laser rod 202 and prevents contact of the laser rod 202 with diode pump sources 208. Pump radiation is produced by the diode arrays 208 which may be coupled into the laser rod 202 by a refracting means 210 (e.g., lenses) or other reflecting means. The coolant fluid removes heat from the laser rod 202 through convection by flowing the fluid 206 over the surface of the laser rod 202. Heat is conducted through the laser rod to the surface. This conduction establishes a large temperature gradient between the center of the rod and the surface. This gradient causes stress within the material which results in beam distortion and eventually catastrophic failure of the crystal. As a result, the average power available from solid-state lasers is limited by the ability to remove heat from the medium.
A class of solid-state lasers which contained the solid material immersed in a fluid were developed in the 1970's and 1980's. These so called “immersion lasers” were flashlamp-pumped and had the solid-state laser material shaped into various forms. For example, as described in U.S. Pat. No. 3,735,282 issued to Gans, a pulsed laser is described which is composed of a segmented Nd:Glass rod immersed in a liquid which is index matched at the emission wavelength. The liquid consists of brominated acyclic hydrocarbon mixed with acyclic alcohol. The segmented rod is immersed in a thermostatically controlled housing surrounded by a helical arc lamp containing xenon or krypton. Gans focuses on a particular type of index matching fluid containing OH and bromine groups to prevent ultraviolet hydrolysis of the index matching fluid.
A similar laser architecture can be found in U.S. Pat. No. 3,602,836 issued to Young, which teaches the use of a segmented laser rod immersed in a coolant fluid. Young focuses on meniscus-shaped segments of zero lens power spaced apart a sufficient distance to permit free passage of sufficient coolant. In U.S. Pat. No. 3,621,456 issued to Young, the use of parallel discs containing reflective surfaces is described. In U.S. Pat. No. 3,487,330, issued to Gudmundsen, a laser arrangement is described in which the flashlamp is enclosed by segmented laser material. In such a system, the coolant flow would be directed inwardly across the segmented laser materials so that the unheated coolant first crosses the laser material and then cools the lamp. Additionally, Gudmundsen also describes that the flashlamp is placed adjacent to the segment laser material. Gudmundsen primarily focuses on various coolant flow geometries for cooling the laser media and the flashlamp.
A packed bed laser composed of solid state glass lasing elements is described in U.S. Pat. No. 4,803,439 issued to Ryan, et al. Ryan et al. teach the use of lasing beads which are packed within a laser cavity to be in contiguous contact with each other. The laser beads are to be of the order of 1 cubic millimeter in volume to facilitate the packing of the glass lasing beads contiguously into the laser amplifier cavity. A cooling fluid is pumped through and in between the lasing beads while the beads are fixed in space. Also, a phase conjugate mirror is required to cancel the optical distortions associated with the lasing medium.
All of these solid-state laser systems including the so-called immersion lasers are limited in average power output by heat removal from the gain media. The difference between the energy of the pump photon and the emission photon is referred to as the quantum defect and left in the crystal as heat. For example, Nd:YAG is pumped by laser diode radiation at 808 nm (photon energy =1.53 eV) and emits laser radiation at 1064 nm (1.17 eV), the quantum defect of 0.36 eV appears as heat within the medium following lasing. This heat must be removed or will terminate lasing by thermally populating the lower laser level (e.g., Yb:YAG) or eventually resulting in catastrophic failure of the laser crystal by thermal stresses associated with the temperature gradient across the crystal. Severe beam distortion and depolarization resulting from the temperature dependence of the refractive index and stress birefringence occur far below the limit of thermal stress induced fracture. Heat is commonly removed by flowing a coolant across the laser material. Alternate heat removal methods designed to address the problem of thermal stress and beam distortion have led to a variety of laser designs, such as thin-disk and zig-zag slab solid state lasers.
What is needed is a laser device in which the advantages of a solid-state gain medium (e.g., diode-pumping, high power density, etc) can be realized but which is not limited in average power output by thermal stress.